1. Field of the Invention
The present invention relates to a planar optical waveguide array module which is used as a terminal for transmitting high-speed optical signals using optical waveguides as optical wiring media between chips or between boards included, for example, in a data processing device or in different data processing devices.
2. Description of the Related Art
In the field of information and communication, the environment for exchanging large-volume data at high speed using an optical means has been rapidly enhanced in recent years. Up to the present, optical fiber networks such as backbone networks, metro networks, and access networks for relatively long distances, i.e. for several kilometers or longer, have been expanded. From now on, it will be effective to expedite the introduction of optical fibers as transmission lines in order to allow large-volume data to be transmitted without delay over very short distances, too, for example, between transmission devices (several meters to several hundred meters) or even within a device (several centimeters to several tens of centimeters).
When optical wiring is employed for data transmission between transmission devices, for example, routers and switches, or within one of such devices, a high-frequency signal received by one of such devices via an optical fiber, for example, from Ethernet is inputted to one of several line cards connected to a backplane. Signals inputted to such line cards are sent to a switch card via the backplane to be returned to the line cards after being processed by an LSI of the switch card. In existing cases, signals are transmitted from such line cards to a switch card via a backplane at a rate of 300 Gbits/s or higher. If electrical transmission lines are to be used, such a high rate of signal transmission requires ten or more lines, as it is necessary to divide the transmission not to exceed a transmission rate of 1 to 3 Gbits/s per line with transmission loss taken into account.
In addition, using high-frequency transmission lines makes it necessary to prepare waveform shaping circuits and measures against reflection and crosstalk between lines. As communication systems grow larger in capacity making it necessary for each device to process data at a rate of Tbits/s or higher, severer problems will be posed, for example, concerning the number of electrical transmission lines to be used and crosstalk between lines. If optical transmission lines are used between boards included in intra-device line cards, a backplane, or a switch card, or between intra-board chips, high-frequency signals can be transmitted at a rate of 10 Gbits/s or higher per line with a small transmission loss. In this case compared with cases where electrical transmission lines are used, the number of lines required can be reduced and it becomes unnecessary to take measures against crosstalk between lines. Thus, using optical transmission lines as described above is a promising approach. Besides routers and switches mentioned above, video devices like video cameras and other consumer devices such as personal computers and cell-phones will also be required, as they come to offer higher image definition, to be capable of high-speed, large-volume image signal transmission between their monitors and terminals. Using electrical transmission lines for such high-speed, large-volume signal transmission will make problems such as transmission delays and noise more conspicuous. To avoid such problems, using optical transmission lines is a promising approach.
To realize high-speed optical interconnection circuits as described above and apply such circuits for inter-device and intra-device signal transmission, it is necessary to realize optical modules and circuits which can be fabricated by an economical means and which excel in terms of performance, compactness, integration, and mountability. Under such circumstances, a compact, high-speed planar waveguide module formed by integrating optical parts and optical waveguides, which are, as optical wiring media, less expensive and more advantageous in achieving high integration density than existing optical fibers, has been proposed.
FIG. 9 shows a basic configuration of a planar lightwave circuit (PLC) module, shown as an example of an existing planar optical waveguide module, including optical parts such as optical elements and optical waveguides mounted on a same substrate. In the configuration, such optical parts as optical elements 101 and 103 (for example, a laser diode and a photodiode) and a filter 102 can be integrated on a platform substrate 100. Therefore, the number of parts required can be reduced and the module can be made smaller. Since optical axis alignment is performed by a passive alignment method, that is, the optical axes of optical parts are aligned when the optical parts are mounted on a platform substrate 100, the number of part mounting steps to be performed in fabricating the module can be reduced.
Another example of an existing type of a planar optical waveguide module is disclosed in JP-A No. 2005-292379. The module includes an optical element array mounted on a substrate and a discrete film optical waveguide array optically connected to the optical element array. In the module, the film optical waveguide array is fixed to a support member provided on the element mounting substrate by concave-convex fitting. To make the concave-convex fitting possible, concaves and convexes are formed on the film optical waveguide array using a transfer substrate. This simplifies the optical module fabrication process and reduces the cost of the optical module.
In the case of the PLC module shown in FIG. 9 as an example of an existing type of a planar optical waveguide module, the optical axes of optical elements are aligned, while monitoring alignment marks provided on the platform substrate 100, by a passive alignment method. Namely, their axes are aligned based only on their positional accuracy on the platform substrate 100. In such a case, the positioning margins for accurately positioning different optical parts on the same substrate are small, so that it is difficult to secure satisfactory optical performance of the module. Moreover, when a module to be fabricated includes optical elements and optical waveguides for multiple channels, it becomes further difficult to achieve a satisfactory yield of modules securing stable optical contact. The performance of such optical parts to be mounted on a substrate can be evaluated only after all the optical parts are mounted on the substrate. Namely, in the case of the above planar optical waveguide module, inspecting individual optical parts in a stage of mounting on a substrate is extremely difficult. This results in a low optical module production yield.
In the planar optical waveguide module disclosed in JP-A No. 2005-292379, too, a discrete film optical waveguide array is optically connected to an optical element array by a passive mounting method, i.e. by concave-convex fitting the film optical waveguide array to a support member provided on the element mounting substrate. Whereas the method makes module fabrication easy, the part positioning accuracy that is a factor in obtaining stable optical connection between optical parts is dependent on the optical part production accuracy. Hence, there is a limit to enhancing the optical part positioning accuracy. Particularly, to achieve efficient optical connection, for example, between a fine optical wiring with a core diameter of several microns for a single-mode optical waveguide and an optical element, a part mounting accuracy on the order of one micron or so is required. When arrayed waveguides are used, a stricter part positioning accuracy is required.